Therefore, when obtaining pages through the follow_trans_huge_pmd
interface, add the FOLL_FORCE flag to count the pages corresponding to
PROTNONE to solve the above problem.
We really want to avoid the usage of FOLL_FORCE, and ideally limit it
to ptrace only.
Fundamentally when removing FOLL_NUMA we did already assumed !FORCE is
FOLL_NUMA. It means to me after the removal it's not possible to say in a
gup walker that "it's not FORCEd, but I don't want to trigger NUMA but just
get the page".
Is that what we want? Shall we document that in FOLL_FORCE if we intended
to enforce numa balancing as long as !FORCE?
That was the idea, yes. I could have sworn we had that at least in some
patch description.
Back then, I played with special-casing on gup_can_follow_protnone() on
FOLL_GET | FOLL_PIN. But it's all just best guesses.
Can always be added if deemed necessary and worth it.
Here, it's simply an abuse of that GUP function that I wasn't aware of
-- otherwise I'd have removed that before hand.
Signed-off-by: liubo <liubo254@xxxxxxxxxx>
Fixes: 474098edac26 ("mm/gup: replace FOLL_NUMA by gup_can_follow_protnone()")
---
fs/proc/task_mmu.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)
diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index c1e6531cb02a..ed08f9b869e2 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -571,8 +571,10 @@ static void smaps_pmd_entry(pmd_t *pmd, unsigned long addr,
bool migration = false;
if (pmd_present(*pmd)) {
- /* FOLL_DUMP will return -EFAULT on huge zero page */
- page = follow_trans_huge_pmd(vma, addr, pmd, FOLL_DUMP);
+ /* FOLL_DUMP will return -EFAULT on huge zero page
+ * FOLL_FORCE follow a PROT_NONE mapped page
+ */
+ page = follow_trans_huge_pmd(vma, addr, pmd, FOLL_DUMP | FOLL_FORCE);
} else if (unlikely(thp_migration_supported() && is_swap_pmd(*pmd))) {
swp_entry_t entry = pmd_to_swp_entry(*pmd);
Might do as an easy fix. But we really should get rid of that
absolutely disgusting usage of follow_trans_huge_pmd().
We don't need 99% of what follow_trans_huge_pmd() does here.
Would the following also fix your issue?
diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 507cd4e59d07..fc744964816e 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -587,8 +587,7 @@ static void smaps_pmd_entry(pmd_t *pmd, unsigned long addr,
bool migration = false;
if (pmd_present(*pmd)) {
- /* FOLL_DUMP will return -EFAULT on huge zero page */
- page = follow_trans_huge_pmd(vma, addr, pmd, FOLL_DUMP);
+ page = vm_normal_page_pmd(vma, addr, *pmd);
} else if (unlikely(thp_migration_supported() && is_swap_pmd(*pmd))) {
swp_entry_t entry = pmd_to_swp_entry(*pmd);
It also skips the shared zeropage and pmd_devmap(),
Otherwise, a simple pmd_page(*pmd) + is_huge_zero_pmd(*pmd) check will do, but I
suspect vm_normal_page_pmd() might be what we actually want to have here.
Because smaps_pte_entry() properly checks for vm_normal_page().
There're indeed some very trivial detail in vm_normal_page_pmd() that's
different, but maybe not so relevant. E.g.,
if (WARN_ON_ONCE(folio_ref_count(folio) <= 0))
return -ENOMEM;
Note that we're not even passing FOLL_GET | FOLL_PIN. Because we're not
actually doing GUP. So the refcount is not that relevant.
if (unlikely(!(flags & FOLL_PCI_P2PDMA) && is_pci_p2pdma_page(page)))
return -EREMOTEIO;
I'm not sure whether the p2pdma page would matter in any form here. E.g.,
whether it can be mapped privately.
Good point, but I don't think that people messing with GUP even imagined
that we would call that function from a !GUP place.
This was wrong from the very start. If we're not in GUP, we shouldn't
call GUP functions.
--
Cheers,
David / dhildenb
